Tif Sippel comes out of a forward stretch with her back arched. The blonde ponytailed personal trainer has just finished a workout session with a client who can’t make his next session.

But this client isn’t getting off easily: Sippel tells him to use a fitness app on his smartphone. She’s got him — she’s his actual coach and his virtual one.

“All right, I want you to do 20 minutes, download a workout, listen and make sure it’s a total body workout,” Sippel tells him. “And if it’s not hard enough, then push harder, because I need you to sweat.”

The app is called Workout Trainer, created by Mission District startup Skimble. It’s one of the top iOS and Android fitness apps powering the mobile fitness movement that’s providing thousands of photo, video and audio coaching workouts to people all over the world. With over 10 million downloads, a partnership with Harvard Medical School and a feature on the “Google Play: New Me” video for 2013, Skimble is just getting started.

The app comes at a time when 36 percent of U.S. adults are obese, according to the Centers for Disease Control and Prevention. Meanwhile, 17 percent of children ages 2 to 19 are also obese. Maria Ly, Skimble’s 30-year-old cofounder, wants to leverage the capabilities of smartphones to make them part of the solution to a healthier lifestyle.

“I realize that the mobile phone isn’t going to solve everything,” Ly said. “But I think there’s a lot of perks to this health trend.” According to the market research firm Nielsen, 50 percent of Americans have smartphones. That means they have access to the online world — and fitness workouts — right in their pockets. “It definitely lowers the barrier to participation,” said Ly.

One testimonial carries special meaning. After a five-year absence, a friend of Sippel’s resurfaced after contracting multiple sclerosis, which affects the nerves of the brain and spinal cord. He had started using Skimble as his only method of working out. “Oh my God, I downloaded that app and saw you on it,” he told her.

Sippel’s friend has been able to involve his kids and sometimes his wife when he exercises. “Those are the kinds of stories where you think, wow, I’m a part of that,” Sippel said. “An app made someone healthier, and that’s awesome.”

Skimble is competing against other health and fitness apps, like RunKeeper, Daily Burn and Nike Training Club. It ranks 16th of 312 apps on Google Play’s list of the top free health and fitness apps on Android. On the iTunes list for iPhone apps, Skimble comes in at 35 out of 200. The profits are there: In 2010, the health and fitness app market was worth $120 million. Come 2016, it will quadruple to $400 million, according to Mobile Health News.

With Skimble, the fun begins as soon as soon as you tap the blue and green icon on your smartphone or tablet. You select a workout like the “10 Min Wake-Up Call.” Then you choose a digital trainer, and up pop moving photos of real trainers who do the exercises with you, guided by a digital voice.

One exercise in the workout, knees-to-elbows, lasts 50 seconds. “Stand with your feet shoulder-width apart with hands behind your head,” says the trainer. “Bring your right knee up and close to your elbows. Return and repeat on your left side.” The digital trainer alerts you when 15 seconds remain and counts down the last five seconds. Then you’re on to the next exercise.

The workouts can be customized to be casual, moderate or intense, whether you want to build endurance, improve flexibility or lose weight. Right now many of the workouts are free. If users decide to “go pro” for $2.99 a month or $14.99 a year, the personal trainers come to life through HD video with professional audio coaching.

Users who choose the “go pro plus” option can access the workouts on their computer — one way Skimble tries to be competitive in the fitness app realm. Another way to get a leg up: The team of 20 trainers creates new content weekly to keep the workouts fresh.

Most of the app’s users spend at least 20 minutes per session. Ly and her cofounder, Gabriel Vanrenen, chose the name Skimble — a combination of “sky” and “nimble” — because they want Skimblers to be active without any limitations.

The app wouldn’t be complete without a social aspect, and Skimble has forums like Women’s Locker Room, Diet & Nutrition and Sports Injuries & Prevention. Micah Greer, one of Skimble’s workout trainers, manages the forums.

“It’s really cool to see [the users’] questions and answer them,” he said. “As a trainer you can only touch the people of your community or city; with Skimble, it’s a gateway to touch people everywhere.”

Take, for example, Mykal Leal, a native Alaskan who used Workout Trainer for almost a year to tone up and lose weight, curating his own workouts on the app. Or Gail Fonseca, a mother of two who has built up her arm and leg muscles using the app. And Gilbert Rod, a six-month user who has lost over 10 pounds in body fat and 2 inches off his waist, plans to walk in the Relay for Life marathon next year.

Skimble has two more apps: Fitness Flow, a collection of HD exercise videos for Androids, and GPS Sports Tracker, which catalogs the user’s progress in sports such as kayaking, horse racing, martial arts and scuba diving.

The desire to catalog her own activities inspired Ly to create Skimble. She and Vanrenen had taken a year off from their previous jobs to train for the sports they love. At some point while climbing the Leaning Tower in Yosemite and Moon Hill in China, and trekking across the Baltoro Glacier in Pakistan, the two realized they needed a tool to track their adventures.

“We created our initial tools to help ourselves,” said Ly. While combating the pain from their adventures on their off days, Ly, a computer science major, and Vanrenen, a computer engineering major, used their knowledge and their hunger for sports to create a Skimble prototype that mostly consisted of Workout Trainer.

Then they submitted an application to an accelerator for digital health startups called Rock Health. Skimble was chosen to be a part of Rock Health’s first class of startups in 2011, receiving a $20,000 grant. Rock Health incubated Skimble and saw tremendous growth. “We call it home,” Ly said of Rock Health. “You never really leave the nest.”

Ly and Vanrenen understand that fitness trainers can be expensive, that workout DVDs can get stale, and that getting to a gym and paying memberships can be challenging, but they want people off their couches and on their yoga mats and stability balls. And they want their phones to come along.

“Smartphones are really smart,” Ly said. “There’s a lot we can take advantage of at this time.”

Tree of Life is a group exhibition that celebrates the rich cultural diversity and assorted art styles found at Creativity Explored. The title of the show references the universal image of the “Tree of Life,” which manifests in folklore, mythology, and world religions. The “Tree of Life” is globally known by many different names, for example, The Cosmic Tree, The Holy Tree, Ydryssil, the Bhodi Tree, and The Tree of...